Recently, use of renewable energy has been desired from the standpoint of depletion of fossil fuel resources, suppression of greenhouse gas emissions, etc. Renewable energy is generated from various energy sources such as sunlight, hydraulic power, wind power, geothermal heat, tidal power, and biomass. Among these renewable energy sources, sunlight has a large amount of energy that can be utilized and has relatively small geographic restrictions as compared with other renewable energy sources. Thus, a technique for efficiently generating usable energy from sunlight has been desired to be developed and widely used at an early stage.
Examples of the form of usable energy generated from sunlight include electrical energy produced by using a solar cell or a solar thermal turbine, thermal energy obtained by collecting solar energy in a heating medium, and storable fuel energy such as liquid fuel or hydrogen produced by reducing a substance with sunlight. Although many solar cell techniques and techniques for utilizing solar heat have already been practically used, the energy use efficiency is still low and the cost for producing electricity and heat is still high. Therefore, techniques for solving these problems have been developed. Furthermore, although these energy forms such as electricity and heat can be used for compensating for a short-term energy fluctuation, they have problems in that, for example, it is very difficult to compensate for a long-term fluctuation such as a seasonal fluctuation, and the operating ratio of power generation facilities may be decreased with an increase in the amount of energy. In contrast, storing energy as a substance such as liquid fuel or hydrogen is very effective as a technique for efficiently compensating for a long-term fluctuation and for increasing the operating ratio of power generation facilities. Accordingly, this is an indispensable technique for maximizing the energy use efficiency and markedly reducing the amount of carbon dioxide emission in the future.
The forms of storable fuel are broadly classified into liquid fuel such as hydrocarbons, gas fuel such as biogas and hydrogen, solid fuel such as wood pellets derived from biomass and metals reduced by sunlight, etc. Liquid fuel is advantageous from the standpoint of ease of construction of the infrastructure and energy density. Gas fuel such as hydrogen is advantageous from the standpoint of improving the total use efficiency in combination with a fuel cell or the like. Solid fuel is advantageous from the standpoint of storage possibility and energy density. Thus, each of the forms has advantages and disadvantages. However, a technique for producing hydrogen by decomposing water by sunlight has particularly attracted attention because water, which is easily available, can be utilized as a raw material.
Examples of the method for producing hydrogen by using water as a raw material and utilizing solar energy include a photolysis method in which platinum is supported on a photocatalyst such as titanium oxide, this substance is put in water, and light irradiation is conducted to perform charge separation in a semiconductor, thus reducing a proton in an electrolyte solution and oxidizing water in the electrolyte solution; a thermal decomposition method in which water is directly decomposed at a high temperature by using thermal energy of a high-temperature gas furnace or the like or water is indirectly decomposed in combination with oxidation-reduction of a metal or the like; a biological method in which the metabolism of microorganisms, such as algae which utilize light, is used; a water electrolysis method in which electricity generated by a solar cell is combined with a hydrogen production device for electrolysis of water; and a photovoltaic method in which a hydrogen generation catalyst and an oxygen generation catalyst are supported on a photoelectric conversion material used in a solar cell, and electrons and holes produced by photoelectric conversion are used in a reaction in the presence of the hydrogen generation catalyst and the oxygen generation catalyst. Among these methods, the photolysis method, the biological method, and the photovoltaic method are believed to have a possibility of producing a compact hydrogen production device by integrating a photoelectric conversion portion and a hydrogen generation portion. However, in view of the conversion efficiency of solar energy, the photovoltaic method is believed to be one of the most plausible techniques for practical use.
Examples of the hydrogen production device in which photoelectric conversion is integrated with hydrogen generation by the photolysis method or the photovoltaic method have been disclosed. Regarding the photolysis method, for example, PTL 1 discloses a device including a photocatalytic electrode composed of titanium oxide to which a ruthenium complex is adsorbed and a platinum electrode, in which oxidation and reduction of iodine or iron are used. According to PTL 2, an integrated structure is realized by connecting two photocatalyst layers in tandem, connecting a platinum counter electrode, and interposing an ion-exchange membrane therebetween. Regarding the photovoltaic method, a concept of a hydrogen production device in which a photoelectric conversion portion, a hydrogen generation portion, and an oxygen generation portion are integrated with each other has been disclosed (NPL 1). According to this literature, charge separation is conducted in the photoelectric conversion portion, and hydrogen generation and oxygen generation are conducted by using corresponding catalysts. The photoelectric conversion portion is composed of a material used in a solar cell. For example, according to NPL 2, charge separation is conducted on three silicon p-i-n layers, hydrogen generation is conducted in the presence of a platinum catalyst, and oxygen generation is conducted in the presence of ruthenium oxide. According to PTL 3 and NPL 3, an integrated hydrogen production device is produced by stacking a hydrogen generation catalyst (NiFeO) and three silicon p-i-n layers in parallel on a substrate, and further supporting an oxygen generation catalyst (Co—Mo) on a silicon layer.